The present invention relates generally to film thickness measurement systems, and more particularly, to cofocal optical systems for making film thickness measurements on patterned wafers.
Many steps are needed to complete the fabrication of a semiconductor chip and it is often desirable to measure the thickness of thin film layers thereof to view features in the 10 to 100 micron lateral size range. This is typically done by scanning a single point high resolution optical probe across the surface of the wafer and making measurements sequentially.
Typically, a full wafer imaging system requires either a large refractive lens assembly, a parabola-based reflector system, or the use of a spherical vacuum chuck to form the wafer into a shallow sphere. In the case of the refractive and reflective systems, the size of the optical elements approaches the size of the largest wafer that is to be measured. These large optics requirement leads to high cost and the optical elements require careful optical design. Forming the wafer into a shallow sphere provides an excellent way of avoiding high cost optical elements although the deformation of the wafer during chip fabrication may be viewed with suspicion and this practice will not work at all if the ambient environment is a vacuum, such as in an automated ultraclean wafer process line.
Prior whole-wafer film thickness mappers require measurement of multispectral reflectance over a full aperture of the imaging and these mappers generally fall into two categories. In the first category, the entire wafer is imaged onto a NxN pixel CCD array (512.times.512, 1024.times.1024, or 2048.times.2048, for example) to provide a spatial resolution at the wafer plane of approximately 200 microns per pixel. In the second category, a CCD array attached to a conventional narrow-field microscope is used provide high resolution in the 1 to 5 micron per pixel range. However, covering the whole wafer requires mechanical scanning of the entire wafer or optical head.
Accordingly, it is an objective of the present invention to provide for a film thickness measurement system that overcomes the limitations of conventional systems. It is a further objective of the present invention to provide for cofocal optical systems for making film thickness measurements on patterned wafers.